Sunday, July 24, 2016

Can we please agree what we mean by “Big Bang”?

The right answer would be “all of the above.” And that’s not because I can’t tell a point from a grapefruit, it’s because physicists can’t agree what they mean by Big Bang!

For someone in quantum gravity, the Big Bang is the initial singularity that occurs in General Relativity when the current expansion of the universe is extrapolated back to the beginning of time. At the Big Bang, then, the universe had size zero and an infinite energy density. Nobody believes this to be a physically meaningful event. We interpret it as a mathematical artifact which merely signals the breakdown of General Relativity.

If you ask a particle physicist, they’ll therefore sensibly put the Big Bang at the time where the density of matter was at the Planck scale – about 80 orders of magnitude higher than the density of a neutron star. That’s where General Relativity breaks down; it doesn’t make sense to extrapolate back farther than this. At this Big Bang, space and time were subject to significant quantum fluctuations and it’s questionable that even speaking of size makes sense, since that would require a well-defined notion of distance.

Cosmologists tend to be even more conservative. The currently most widely used model for the evolution of the universe posits that briefly after the Planck epoch an exponential expansion, known as inflation, took place. At the end of inflation, so the assumption, the energy of the field which drives the exponential expansion is dumped into particles of the standard model.
Cosmologists like to put the Big Bang at the end of inflation because inflation itself hasn’t been observationally confirmed. But they can’t agree how long inflation lasted, and so the estimates for the size of the universe range between a grapefruit and a football field.

Finally, if you ask someone in science communication, they’ll throw up their hands in despair and then explain that the Big Bang isn’t an event but a theory for the evolution of the universe. Wikipedia engages in the same obfuscation – if you look up “Big Bang” you get instead an explanation for “Big Bang theory,” leaving you to wonder what it’s a theory of.

I admit it’s not a problem that bugs physicists a lot because they don’t normally
debate the meaning of words. They’ll write down whatever equations they use, and this prevents further verbal confusion. Of course the rest of the world should also work this way, by first writing down definitions before entering unnecessary arguments.

While I am waiting for mathematical enlightment to catch on, I find this state of affairs terribly annoying. I recently had an argument on twitter about whether or not the LHC “recreates the Big Bang,” as the popular press likes to claim. It doesn’t. But it’s hard to make a point if no two references agree on what the Big Bang is to begin with, not to mention that it was neither big nor did it bang. If biologists adopted physicists standards, they’d refer to infants as blastocysts, and if you complained about it they’d explain both are phases of pregnancy theory.

I find this nomenclature unfortunate because it raises the impression we understand far less about the early universe than we do. If physicists can’t agree whether the universe at the Big Bang had the size of the White House or of a point, would you give them 5 billion dollars to slam things into each other? Maybe they’ll accidentally open a portal to a parallel universe where the US Presidential candidates are Donald Duck and Brigitta MacBridge.

Historically, the term “Big Bang” was coined by Fred Hoyle, a staunch believer in steady state cosmology. He used the phrase to make fun of Lemaitre, who, in 1927, had found a solution to Einstein’s field equations according to which the universe wasn’t eternally constant in time. Lemaitre showed, for the first time, that matter caused space to expand, which implied that the universe must have had an initial moment from which it started expanding. They didn’t then worry about exactly when the Big Bang would have been – back then they worried whether cosmology was science at all.

But we’re not in the 1940s any more, and precise science deserves precise terminology. Maybe we should rename the different stages of the universe that into “Big Bang,” “Big Bing” and “Big Bong.” This idea has much potential by allowing further refinement to “Big Bång,” “Big Bîng” or “Big Böng.” I’m sure Hoyle would approve. Then he would laugh and quote Niels Bohr, “Never express yourself more clearly than you are able to think.”

43 comments:

Science is such a precision-driven endeavor, yet so much of our terminology is ambiguous. We as a community really ought to think more about how we are communicating our ideas, not just between our fellow scientists, but with Mr. and Mrs. Average Citizen, as well.

I try to use (and advocate using) "Big bang singularity", "Big bang theory", and "Big bang fireball" without ever leaving off the third word.

And I almost never say things like "the universe started 13.8 billions years ago" or "1 billion years after the big bang," Though this often leads to less efficient sentences, sometime specifying what exactly the 13.8 billion years is measuring can be useful.

"universe" or "Universe" is almost as bad nowadays. I routinely am sorting between several meanings of this. Using "observable universe" whenever that is what one really means is a good start, though it tends to go unnoticed...

There is yet another definition: the Big Bang is not a discrete event but the period of expansion in which we are now living. Astronomers oscillate between this definition and the singularity definition.

I think it's useful to recall a presiding creative ethos at the period that Big Bang 'theory' emerged. Dada winding down somewhat, a budding surrealism, a still-thriving cubism with its varied esthetic tangents, and of course Arnold Schoenberg -- with his self-professed classicism and peculiar contempt for words in toto (along with a deep-seated fear of the number 13). Did Einstein make public his position, if any, on the Miros, the Becketts, the Mondrians? The imprecision in terms of definition quite arguably has theosophy, cartoons 'as such,' and nihilism to address, to somehow reckon with, before terminology can reembody any true sanctity of precision. Thks for the Bohr quote!

One taxonomy of various definitions of the Big Bang that I found useful is at https://profmattstrassler.com/2014/03/26/which-parts-of-the-big-bang-theory-are-reliable/, I'd be interested to get your assessment of it.

According to my understanding, belief that some "Big event(!)" happened 13.8 B years ago is quite well established.It could be the first beginning or more likely end of a previous cycle and beginning of a new cycle.The debate is only about what happened in the first second or at worst first minute.As far as I am concerned this is a great advance and we should not have any problem in telling non scientists something like this. Whether inflation took place and how, although important, are secondary matters.Of course it is important to understand how matter was created from vacuum by quantum fields and at what instant during the first second. But perhaps in due course, there will be good models.

Its just a fixed reference in a talking point. It gives people an idea to hang onto ... especially those who can't wrap their heads around the phenomenon or are just casually browsing the subjectmatter. I understand that the correct mental picture is important but for most people you get 15 minutes and out and they've moved on. Sad but true.

I believe the most sensible definition is to define the Big Bang to be the singularity of the classical Big Bang theory, even if we believe that this theory breaks down close enough to this singularity and becomes nonsingular. The answer to any question on what happens "at the Big Bang" is undefined since the Big Bang is not really a point in the spacetime manifold. However, any point has a finite-time geodesic to the Big Bang, so it is well-defined to talk about what happens at time t after the Big Bang. The success of the Big Bang theory comes from that fact that it provides a model for what the world looks like at time t after the Big Bang, which becomes progressively more accurate as t gets smaller. At sufficiently early events the theory stops being accurate, and we stop being able to define "distance to Big Bang" accurately. However, this vagueness is at a tiny scale compared to most events in the convention Big Bang cosmology. It's unlikely to cause problems if we say things like "Neutrinos decoupled one second after the Big Bang" since the vagueness is at a scale much smaller than one second.

As an analogy, we can talk about the tip of a needle even though we know that the needle's shape is not really singular, but becomes smooth at its end at sufficiently high precision. We just need to remember that point is only defined to a finite precision, and to stop thinking of it as point whenever you need more precision.

You've obviously hit a nerve. Anthony Aguirre says one should say 'observable universe' when that's what one means, yet how many do mean that? It begs continuation and contrast -- observable relative to what? I like George Musser's definition; it accounts for time.

First of all, many thanks to Sabine for her time and effort in writing this piece. You've spelled things out well.

I am a little tentative about posting because discussions online are sometimes quite difficult and can lead to confusion, unless we are extremely careful.

Let me see whether we are singing from the same sheet of music. Do you agree that the trouble with 'Big Bang' is that it evokes a physical picture in the mind? As you have written, physics breaks down in regions of high temp and high density, so that we cannot speak intelligently about what happened "very early" if you will.

If I am reading you right, then I guess you are would agree with a move away from overly specific and binding ideas and towards more general ones. Thus you might agree thatwe replace 'the big bang' with a simpler term such as 'the cosmic origin.' This is less suggestive and more neutral term, yet not so general that it omits the notion of a beginning, a notion we wish to investigate further, etc.

I prefer not to ascribe a size to the Big Bang, even one that cites the visible universe. This is mainly because GR Friedmann solutions typically have no spatial boundaries, so that providing a size may cause the reader to visualize a small sphere that explodes into an empty vacuum, a decidedly Newtonian visualization.

"At the Big Bang, then, the universe had size zero and an infinite energy density. Nobody believes this to be a physically meaningful event. We interpret it as a mathematical artifact which merely signals the breakdown of General Relativity."

If the universe is infinite, it was also infinite at the big bang. Size zero would (classically) happen in a spatially finite universe.

"The currently most widely used model for the evolution of the universe posits that briefly after the Planck epoch an exponential expansion, known as inflation, took place. At the end of inflation, so the assumption, the energy of the field which drives the exponential expansion is dumped into particles of the standard model. Cosmologists like to put the Big Bang at the end of inflation because inflation itself hasn’t been observationally confirmed."

This is Max Tegmark's definition. As a believer in eternal inflation, for him the big bang isn't when something started, it is when something stopped.

"I find this nomenclature unfortunate because it raises the impression we understand far less about the early universe than we do. If physicists can’t agree whether the universe at the Big Bang had the size of the White House or of a point, would you give them 5 billion dollars to slam things into each other?"

It is interesting to see how loosely the term Big Bang is used by different communities.

From a general relativity perspective, the simplest cosmological solution reaches zero spatial volume in finite proper time, from which we would conclude that the classically correct answer to the question is “a point". Now you may argue that quantum gravity effects take over before that. A sensible moment where they could become relevant is when the universe is close to Planck energy density (this is also consistent with growing evidence from loop quantum gravity). But Planck energy density can be reached at any spatial volume, so there is no natural scale for the "big bang replacement" prescribed by such effects.

"From a general relativity perspective, the simplest cosmological solution reaches zero spatial volume in finite proper time, from which we would conclude that the classically correct answer to the question is “a point"."

Not sure what you mean by "simplest cosmological solution".

The scale factor goes to zero. But for k<1, the universe is spatially infinite. So a physical distance is 0 times infinity. What is the proper answer? IIRC, it is infinity in this particular case.

I should have been more precise. For me, the simplest solution would be a k=0 FRW universe with 3-Torus topology of the spatial slices. There, you have finite coordinate volume x 0 for the physical volume when the scale factor vanishes, which seems to leaves little room for interpretation. But then you wonder how a torus can suddenly be contracted to a point... Not sure what to say about that.

I agree that the situation is less clear if you have an infinitely extended universe. But from a quantum gravity point of view, it is anyway satisfying (at least to me) that a universe would bounce once Planck density is reached. Then, the extend of the universe can be finite or infinite depending on your concrete model, but this is irrelevant to determine whether you are close the event replacing the big bang or not.

"Lemaitre showed, for the first time, that matter caused space to expand"

Another great post. That quote stopped my mind in its tracks. I dumbly thought I understood that the Cosmological Constant/Dark Energy made space expand and that matter causes space to contract (e.g., "The Big Crunch"). It turns out I know nothing. Which is good to know (if you know what I mean).

This is a very common misunderstanding, which comes about because we are used to think of gravity as matter affecting other matter, not as matter affecting space. Matter attracts other matter, but matter (of any type) causes the universe to expand. Hence, while the universe expands, matter clumps and forms structures like galaxies and solar systems etc - these two processes (clumping and overall expansion) are not in contradiction with each other.

The cosmological constant (or dark energy more generally) is necessary for an *accelerated* expansion - the point being that you cannot get such an accelerated expansion with normal matter in any way.

It's one of these things that sounds foggy in words but becomes brilliantly clear if you look at the equations. The relevant equations are the Friedmann equations. The first Friedmann equation tells you that all energy density causes a time-dependence of the scale factor (a). The dot is a time derivative, rho is the energy-density. You can ignore the term with "k", just set it to zero.

The second Friedmann equation is for the acceleration of the scale-factor (hence two dots). You see that it's sourced by both the energy-density and the pressure, but more importantly, the cosmological constant enters there. Best,

If you don't say the big bang is the process by which the non-existent state of the universe, psi=0, suddenly stopped being equal to zero, then you should just call it what it is: the fudge factor needed to make the current theories work with the modern data.

The value of da/dt appears only squared in the Friedmann equations, so a positive density can correspond to either an increasing or a decreasing scale factor. Indeed the classical Friedmann equations admit solutions that consist of expansion followed by contraction. During the contraction phase, the increasing density corresponds to increasingly negative values of da/dt, so in these circumstances matter “causes” the universe to contract. Granted, current observation suggests that our universe won’t have a contraction phase, but we can’t infer this from the Friedmann equations. The equation tells us only that the density is related to the square of the derivative of the scale factor, but the derivative itself may be either positive or negative.

By the way, for a given value of Hubble’s constant H (and assuming zero cc), if the value of (8pi/3)rho exceeds H^2 then this "causes" k to be positive, which “causes” the classical Friedmann universe to eventually stop and contract. I think this is the kind of reasoning that led people traditionally to say (somewhat loosely) that a higher density of matter would “cause” eventual contraction. Of course we could take different parameters as “given” and infer different “causalities” between the other parameters, so this isn’t a very robust sense of “causation”.

Save the "Big Bong" for those of us in Colorado who know who to use it. ;)

Seriously though, over precision in terminology is overrated. What is wrong with considering the "Big Bang" to merely refer to an event roughly 13.5 +/- billion years ago in which matter and energy was highly concentrated from which our current universe has expanded ever since and using other less familiar terms to describe more precise versions of the event.

In other words, there is nothing wrong for having a term for a genus of events that encompasses various species, even ones that look very different from each other. If cabbage, broccoli, cauliflower, turnip, rapeseed, mustard, radish, horseradish, cress, wasabi, and watercress can all rest harmoniously as different kinds of Brassicaceae, why can't all manner of diverse theories co-exist under the general descriptor of the "Big Bang" whose very colloquial register doesn't imply any great precision.

The reason I am not approving your comments is not that I am "censoring" you, but that I've already wasted more than enough time trying to clear up your misconceptions. Your constant insistence that something I've written is supposedly wrong because you don't understand it is both useless and uninteresting.

Within cosmology I don't hear much mention of the "big bang" at all. The "big bang model" has been refined to the LCDM model. For "big bang singularity" I'd use "cosmological singularity". We do refer to the "hot" or "thermal big bang", which refers to the period of thermal equilibrium, not to a moment in time.

On the question of matter "causing space to expand", it depends on what you mean by "space". If you mean empty space (vacuum), then for every expanding congruence you show me, I can find one in the same region that contracts or oscillates or whatever. In other words, the state of expansion of empty space is coordinate dependent.

In the 1st Friedmann equation (ie 00 Einstein equation), "H" measures the expansion of the matter-comoving worldlines. So the statement should be that matter causes matter to expand (or contract for the other branch). Or more generally that, due to gravity, matter must be dynamical (in the absence of finely-tuned constructions). Which is entirely reasonable.

As I wrote, if quantum gravitational effects are pronounced it's questionable that speaking of a size makes sense at all. But that's not an answer I commonly read in the pop sci literature, or do you? Best,

What I actually had in mind was conformal/scale symmetry as in Penrose's cyclic model or in asymptotic safety where distances and sizes (and times) loose their (operational) meaning. (A spectacular manifestation of that being Penroses' "conformal matching" of an old and a young universe).

But, yes, these are probably not the kind of things one usually finds in pop sci literature :-)

As narratives the various big bang stories are not very satisfying and perhaps they are missing half of the action. Was there something formative before spacetime itself? We have strong quantum fluctuations, but fluctuating about what norm? Did energy burst from the egg alone or entwined with its counterpoise, the vestige of its shell, a congenital dynamic constraint? Gradients arise only with counter currents.

For your normal person the BB is the point at which the Universe came into being. It arose from nothing and there was a massive explosion and everything was created and hence this is what explains the expanding Universe. Cosmologists, particle physicists etc only have themselves to blame that it is viewed this way.

I know that singularities (big bang and black holes cause issues due to being size zero classically which makes no sense) make no sense classically but quantum mechanically might mean something differently but as we don't have a quantum gravity as yet then its all unexplained as yet.

I understand the first lesson in research is "when you're stuck, examine your premises" (- and I think we are stuck "out of" quantum gravity).

Then what is a really shocking to me in those discussions is the seemingly absolute belief in the fact that energy is conserved (where another post on this blog states there is no reason for this) - leading to its high concentration ~13.8 billion (of our) years ago. After all, the Big Bang maybe about creation; not the one of creationists but something new in a realm that we do not understand.

How can metric appear out of its own thin air, without energy emerging at the same time? Why should energy be the cause of expansion - or the opposite? Why not both the effects of the same cause?

@akidbelle Noether's theorems couple symmetries with conserved properties. Conservations of mass-energy; linear and angular momenta respectively arise from homogeneity of time; homogeneity and isotropy of space. Earth is a gravitational potential. Time is not homogeneous with altitude. Organisms are not spatially homogeneous. Graphite is anisotropic. (alpha-Quartz enantiomorphs falsify the Equivalence Principle?)

"f we run the Universe backwards in time what happens when we get to the beginning, does all the Universe go into a singularity or not ?"

No-one knows with absolute certainty, but very probably not. At such high densities, quantum effects have to be taken into account. It's just that, without a theory of quantum gravity, no-one knows how to do this.

Usually, when a theory predicts a singularity, what actually happens is that the theory is no longer valid in that regime.